Chemical and mechanical alterations of SiC nicalon fiber properties duringthe CVD/CVI process for boron nitride

To improve the interfacial properties in SiC/SiC composites, BN is an appro priate interphase material to control the fiber/matrix bond. Unfortunately, the gaseous phase (NH3,BF3,HF,Ar) used to deposit BN acts aggressively upo n Si-C-O (ex-PCS) Nicalon fiber surfaces, and weakens that bond through the formation of a complex interfacial sequence (SiO2/C), which actually contr ols the localization of debonds. The reactions between each gas involved an d the fiber surface have been studied. Further, if the fiber surface consis ts of SiC or any silicon-containing compound, the BF3 gas reacts through a substitution of the silicon by boron in the initial fiber composition. Then , the surface evolves from a (C,O,Si) composition to a (B,C,O,Si) glassy la yer. Such a reaction occurs mainly before the BN nucleation, and it alters the reinforcing potential of fibers in composites. This boron-containing gl ass is shown to be very unstable in the presence of HF gas (the main reacti on product). (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.